This invention relates generally to improvement of the engineering properties of hydraulic cements and, more particularly, to set control compositions having a modified gypsum component.
It is well-known in the cement/concrete industry that cementitious materials are prepared by combining the sintered product of the oxides of calcium, silicon, iron and aluminum (commonly abbreviated C, S, F and A, respectively, in the industry) with a calcium sulfate material. Subsequent addition of water to the cement provides a workable material which hardens and gains strength as hydration proceeds to completion. Varying the proportions of the initial oxide and sulfate materials and the process conditions affects the physical properties and performance of the resulting mortar or concrete. More specifically, these properties and subsequent performance are, in large part, determined by the relative amounts of, interaction between and hydration of the sintered oxide products: tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite (abbreviated C.sub.3 S, C.sub.2 S, C.sub.3 A and C.sub.4 AF, respectively, in the industry). Generally, C.sub.3 S contributes to early and later compressive strengths, while the contribution of C.sub.2 S is limited to later strengths. C.sub.3 A contributes to early strength, but is sulfate susceptible. While C.sub.4 AF is also sulfate susceptible and adds little to early strength, it does enhance later strength. It is understood that while these four oxides and their products of sintering are those basic to the preparation and function of cementitious materials, other chemical components will also be present depending upon the source and/or identity of the raw materials used.
As mentioned above, the sintered product, clinker, is finally ground with an appropriate amount of a calcium sulfate material, usually gypsum. The resulting mixture is a hydraulic cementitious material, of which portland cement is a well-known representative. The principal function of the sulfate material is to control the set time of the cement upon mixing with water and through the course of hydration. Although the final setting of most cement materials is predominately influenced by the reaction of the C.sub.3 S phase, it is generally believed that the particulate surfaces of this phase can be quickly covered by hydrated reaction products, slowing additional reaction and development of early strength. In contrast, initial reaction of the aluminate phases with water is immediate and characterized by vigorous formation of crystalline hydrate. Unless aluminate hydration is moderated an undesirable flash set condition occurs. In the presence of a calcium sulfate material, among other effects, an insoluble sulfoaluminate coating forms on the particulate surfaces and slows the rate of hydration.
In contrast to flash set, false set conditions--initial, rapid rigidity--can also occur. During grinding with the clinker material, calcium sulfate dihydrate can be dehydrated to hemihydrate or soluble anhydrite forms. When the cement is mixed with water, rehydration can form an initial mass of dihydrate crystals, causing an initial rigidity, which appears to resemble actual set conditions.
False and flash set control conditions illustrate the importance of a sulfate component to a cementitious material. A set control composition should have a chemical profile which facilitates an appropriate degree and rate of hydration. Furthermore, a set control composition must be compatible with a particular cement to achieve acceptable strengths and/or related performance parameters.
It is desirable, therefore, to have an inexpensive set control composition that is compatible with various cementitious materials, which can be interground with clinker material or admixed with the ground product and that will meet or enhance the various physical, chemical and/or performance standards recognized through out the industry, both here and abroad, especially as they apply to the portland cements as specified under various ASTM, B.S., DIN and equivalent standard specifications.